AUTUNITE & META-AUTUNITE
Autunite and meta-autunite are radioactive, yellowish to yellowish-green, uranium-bearing minerals. They are both hydrous calcium uranyl phosphate minerals - autunite is Ca(UO2)2(PO4)2·11H2O and meta-autunite is Ca(UO2)2(PO4)2·6-8H2O.
Autunite is an alteration product of other uranium minerals and typically occurs in hydrothermal veins and pegmatitic granites. It has a nonmetallic, glassy to pearly luster, varies from pale yellow to yellow to greenish yellow to pale green to dark green to greenish black, has a yellowish streak, is soft (H=2 to 2.5), and has 3 cleavage planes, one of which is perfect (micaceous) and two of which are poor. Autunite crystals are tabular in shape, having squarish to rectangular to 8-sided outlines, and range from transparent to translucent to opaque. Many autunite specimens are finely-crystalline aggregates or microcrystalline crusts. Thin crystals and thin cleavage fragments are flexible (elastic). Autunite fluoresces under ultraviolet (UV) light.
Meta-autunite is a more stable, partial-dehydration product of autunite, and many "autunite" specimens are likely meta-autunite. Meta-autunite has a nonmetallic, glassy to pearly to dull luster, a bright yellow to greenish yellow color, yellowish streak, is soft (H=2 to 2.5), and has one perfect cleavage (micaceous). Unlike autunite, thin crystals and cleavage fragments of meta-autunite are brittle, not flexible. Meta-autunite crystals are tabular squares to pseudocubes to pseudomorphs of autunite. Finely-crystalline to microcrystalline crusts are common. Meta-autunite is also fluorescent under UV light.
Autunite (above & below) from Autun, France (Colorado School of Mines Geology Museum public display, Golden, Colorado, USA).
Meta-autunite from Margnac, Limousin, France (CSM # 69.145, Colorado School of Mines Geology Museum, Golden, Colorado, USA).